Controlled electrochemical functionalization of CNT fibers: Structure-chemistry relations and application in current collector-free all-solid supercapacitors

Abstract

Chemical functionalization of nanocarbons is an important strategy to produce electrochemical systems with higher energy/power density by generating surface functional groups with additional faradaic contribution, by increasing their surface area and correspondent capacitive contribution and by improving compatibility with aqueous electrolytes and other active materials, such as pseudocapacitive metal-oxides. Here we present an electrochemical method to simultaneously swell and functionalize large electrodes consisting of fabrics of macroscopic fibers of carbon nanotubes that renders the material hydrophilic and produces a substantial increase of specific capacitance and energy density in aqueous electrolytes. Through in-depth characterization of the carbon nanotube fibres (CNTF) by Raman spectroscopy, transmission electron microscopy, X-ray photoelectrocn spectroscopy (XPS) and small-angle X-ray scattering (SAXS) we identify various contributions to such improvements, including surface oxidation, tubular unzipping, debundling and inter-bundle swelling. Changes in hydrophilicity of functionalized CNTF are determined by analyzing the dynamics of spreading of polar and nonpolar liquids in the electrode. The extracted contact angles and polar and dispersive surface energy components for different treatment conditions are in agreement with changes in dipole-moment obtained by XPS. Finally, functionalized CNTF electrodes were employed in current collector-free solid flexible supercapacitors, which show enhanced electrochemical properties compared to as-produced hydrophobic ones. 1. INTRODUCTION The superlative properties of nanocarbons continue to fuel the interest in macroscopic architectures that efficiently exploit their "molecular" properties. The exceptional stiffness, charge mobility and electrochemical stability of CNTs, for example, makes them attractive for applications ranging from lightweight composites, to electrochemical charge storage/transfer processes to biomedical applications. 123 In this quest, macroscopic CNT fibres, yarns and fabrics have emerged as attractive systems in which the CNTs associate in long coherent bundle domains that favor inter-tube charge and stress transfer, while also leaving large mesoporous gaps between bundles and thus giving rise to a large porosity. Such structure leads to an unusual combination of bulk mechanical toughness, electrical conductivity and electrochemical stability above that of many metals, combined with a large specific surface area above 250 m 2 /g. These property envelop is partly the reason of the increasing use of CNT fibres as electrodes/current collectors in multiple energy storage, transfer and conversion devices, including those requiring light weight and augmented mechanical properties 4 .

High rate hybrid MnO2@CNT fabric anodes for Li-ion batteries: properties and a lithium storage mechanism study by in situ synchrotron X-ray scattering

“…Free-standing hybrid MnO2@CNT fabrics were prepared by electrodepositing manganese oxide on electrochemically pre-functionalized CNT fiber fabrics from a solution of manganese nitrate in water and ethanol mixture (1:1). 41,50 The FESEM images of the sample at different magnifications are shown in Figure 1a which combine exceptional mechanical toughness and a 29 % increase in specific capacity compared to commercial LIB cathodes, preserved even after tensile fracture. 51 Figure 1c and d show the high resolution TEM images of the MnO2@CNTF hybrid.…”

Origin of the electrocatalytic activity in carbon nanotube fiber counter-electrodes for solar-energy conversion

“…Ozonolysis also removes surface impurities with no detectable effect on the residual catalyst nanoparticles. 66 The degree of functionalization can be readily monitored by Raman spectroscopy: a drop in absolute intensity due to resonance loss, the appearance of the D 0 mode (double resonance induced by defects and disorder) and the increase of the intensity ratio of the D and G bands (I D /I G ) 68 are the main characteristics. Here, I D /I G increases from 0.64 AE 0.08 for the pristine material (1), to 0.80 AE 0.05 (2), and to 1.38 AE 0.05 (3) for the samples functionalised for 2 and 4 minutes, respectively.…”

“…The corresponding changes in surface energy and elemental O/C concentration can be found in references. 56,68 These CNTf samples were transferred onto glass substrates and were applied as electrodes for symmetrical cells with a CNTf/cobalt-based electrolyte/CNTf congurationsee the Experimental section for details. The electrocatalytic activity of pristine and functionalized CNTf-CEs was studied using linear sweep voltammetry (LSV).…”

Cold‐Resistant Nitrogen/Sulfur Dual‐Doped Graphene Fiber Supercapacitors with Solar–Thermal Energy Conversion Effect

“…With the increasing applications of wearable and portable electronics, including electronic textiles, communication devices, and robotic sensory skins, it is imperative to develop high‐performance energy storage devices adapted to various ambient conditions [1–4] . Although all‐solid‐state supercapacitors are competitive alternatives because of their fast charge/discharge rates, high power densities, and good cycle performances, their unsatisfactory energy densities and easy freezing of electrolytes at low temperatures restrict their wide application [5–10] . Recently, graphene fiber‐based electrode materials have drawn significant interests owing to their structural flexibility, high conductivity, ease of functionalization, and lightweight features [11–17] .…”

“…[1][2][3][4] Although all-solid-state supercapacitors are competitive alternatives because of their fast charge/discharge rates, high power densities, and good cycle performances, their unsatisfactory energy densities and easy freezing of electrolytesa tl ow temperatures restrict their wide application. [5][6][7][8][9][10] Recently,g raphene fiber-based electrode materialsh ave drawn significanti nterests owing to their structural flexibility, high conductivity,e aseo ff unctionalization, and lightweight features. [11][12][13][14][15][16][17] However,t he irreversible agglomeration of graphenes heets as ar esult of their van der Waals interaction reducest he ion accessible surface area and slows down the ion diffusion rate in fiber electrodes.…”